Refactor isTetheringSupported() calls

Tethering currently wants access to complex isTetheringSupported
logic that is only available in ConnectivityService. Instead of
trying to access that via ConnectivityManager, pass this capability
in to Tethering directly, in the TetheringDependencies object.

Also:
    - ConnectivityManager is only a source of static constants now,
      so "import static" all the constants that are actually used.

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net works
    - manual USB towards WiFi tethering works
Bug: 68951715

Change-Id: Ia64faaadefb4a5d84a50da98bdebd544b6fda101